There are various types of electronic displays, for example reflective displays such as electrophoretic, electrowetting, electrofluidic and photonic, or emissive displays such as LCD. Such electronic displays may be incorporated in an electronic document reader which is a device such as an electronic book which presents a document to a user on a display to enable the user to read the document.
When power is removed from emissive displays (such as LCD, OLED and Plasma) they revert to an off-state. This state is known and any colour can be driven accurately from this starting point. Reflective displays, e.g. electrophoretic displays, differ since they retain the last image that was written to them. Therefore, the display must be unwritten before it is rewritten. An electrophoretic display is a display which is designed to mimic the appearance of ordinary ink on paper and may be termed electronic paper, e-paper and electronic ink. Electrophoretic display media is unlike most display technologies.
Typically the image displayed on an electrophoretic display is greyscale (or monochrome). Displaying coloured documents using a black and white display often results in the loss of important information. Colours that were used to distinguish different parts of content can be rendered to grey levels that are so similar that it is difficult to tell the difference. Similarly, coloured text may be converted to a grey level so light it makes it difficult to read.
The table illustrates the maximum contrast ratio, number of unique colours and typical resolution of some information displays and printed media.
Typical contrastNumber ofTypicalratiocoloursresolutionLCD:100 s to 1000 s:116.78 million (24 bit)~100ppiDigitally printed86:116.78 million (24 bit)400dpiphotograph:Magazine:55:14 (CYMK)~300dpiNewspaper: 6:14 (CYMK)~200dpiE-paper:8 to 10:116~150ppi
As set out above, e-paper display displays have a unique challenge over some other display technologies; they neither support the number of colours that an LCD has, neither do they have the resolution that printed media have to enable efficient “half toning” or “dithering”. When displaying content, originally designed for colour display or print, these deficiencies can lead to a degraded user perception of quality, and in the worst case information can easily be lost.
These challenges are not unique to e-paper displays and there are other displays having limited colour gamut, limited number of colours/shades of grey and/or limited dynamic range and/or contrast. Where these challenges exist, the applicant has thus recognised the need for an improved display, particularly but not limited to an electrophoretic display. Such displays are termed “limited colour” displays. The improvement may relate to the processing of data representing the image which may be done within an electronic document reader itself or in a separate electronic device, e.g. a laptop, mobile phone etc.